This invention relates to a method for producing a continuous liquid spray curtain capable of uniformly covering essentially the entirety of a moving surface, without substantial streaking thereof.
Prior art systems have been provided for spraying a liquid onto a moving surface. For example, a plurality of hydraulic nozzles can be employed for liquid spraying, the number of nozzles employed being determined by the width of the surface to be sprayed. However, hydraulic nozzles emit a spray in a circular or elliptical pattern. This causes nonuniform coverage of the moving surface because the respective sprays emanating from adjacent hydraulic nozzles are difficult to interface one with the other over the entire width of the moving surface. Thus, streaking results due to these respective oversprayed or undersprayed areas. Streaking is a particular problem in certain applications, such as, for example, spraying a creping adhesive onto a cellulosic web, or onto a thermal drying cylinder, since nonuniform adhesion of the web to the thermal drying cylinder results in a nonuniformly creped sheet having substandard physical properties. Furthermore, the dried, creped web will not wind evenly into a parent roll on the papermaking reel if creping is nonuniform. This will lead to substantial problems when the parent roll is converted to product.
Another serious problem associated with certain nozzles, such as hydraulic nozzles, is plugging of the nozzle tips. Plugging terminates liquid flow causing widespread streaking to occur due to the aforementioned nonuniform spray application. Hydraulic nozzles operate at a relatively high solution flow rate. Therefore, if an adhesive is the liquid material sprayed, a total solids level must be selected at a given liquid flow rate which will not provide too large an amount of adhesive to be sprayed onto the moving surface. This will cause a boardy sheet to be formed. Thus, a lower, overall total solids liquid must be employed at a higher total solution flow rate in order to supply the prescribed amount of solids add-on to the surface to be sprayed. This results in the use of much higher water consumption level, as well as a substantial increase in the thermal energy required for drying purposes.
The exit velocity of the liquid in a hydraulic nozzle system determines the requisite degree of atomization of the liquid. In the case of a hydraulic nozzle, the liquid exit velocity is relatively high. The exit velocity is primarily a function of the liquid supply pressure. A high liquid supply pressure presents severe operating hazards to equipment and personnel.
Another approach in spraying a liquid onto a moving surface is the use of sonic nozzles. These nozzles typically spray particles of a smaller, more uniform size particle distribution than those produced by hydraulic spraying. A discussion of this type of spraying appears in pending application U.S. Ser. No. 99,041, which is assigned to the assignee herein. One of the major problems which can result from the use of a plurality of sonic nozzles for spraying onto a moving surface is that the finer the spray which is produced, the lower the momentum of the spray particles. This, in turn, reduces the effect of penetration by the spray particles of the boundary air layer between the nozzle and the moving surface, resulting in a significantly higher level of spray migration and a lower solids addition to the moving surface. Furthermore, the same coverage problems associated with hydraulic nozzles are present herein because of the circular spray patterns produced by each adjacent sonic nozzle. Finally, the sonic nozzles exhibit plugging problems similar to those described above for hydraulic nozzles.
Other prior art systems have attempted to provide a plurality of sprays from a common source. U.S. Pat. No. 1,888,791 to Cole, for example, describes an apparatus which discharges liquids through jets 1-3. The discharge liquid intersects air streams 4-6 outside the discharge orifices at a substantially maximum angle with respect to the central axis of the liquid jet so that the air streams impede the progress of the liquid jet flow and creates a back-pressure. Any change in the air velocity or impingement angle will change the back-pressure. For example, any increase in the back-pressure, such as caused by an increase in the air velocity, will result in a decrease in both the liquid velocity and in the amount of liquid sprayed. Thus, since the velocity and amount of liquid sprayed, respectively, will be changed by changes in the back-pressure, spray uniformity in both the lateral (coverage) and longitudinal (uniform rate) directions will be difficult to maintain. Therefore, higher relative liquid pressures and velocities then desired must be maintained with respect to the Cole apparatus in order for the system to function since small variations in either the air or liquid discharge velocity will result in substantial changes in the lateral and longitudinal spray pattern. This results in the aforementioned streaking, uniformity, and coverage problems. Finally, the air stream in Cole emanates from individual sets of jets 4-6. Therefore, the air stream is discontinuous over the entire longitudinal extent of the apparatus. A discontinuous air stream will create a discontinuous spray flow pattern, resulting in streaking and nonuniform coverage of the surface being sprayed.
With respect to certain moving surfaces, such as cellulosic webs, and the like, a nonuniform moisture profile typically exists in which the edges of the webs are much drier than the central portion. Coverage of these webs with moisturizing liquids to a desired moisture level can be accomplished by the addition of water to increase the moisture level at the edges of the web. Some prior art systems, such as sonic nozzles, attempt to correct this problem by changing the flow rates of a plurality of individual sonic nozzles in a given system so as to alter the moisture profile of the web. Instead, the system provides a random, nonuniform, uncoordinated spray pattern.